Continuing Education

Colin Fowler charts the development of the progressive power lenses, from the early 20th century patents to the latest individualised products (CET module C3737)

Progressive power spectacle lenses have now been available commercially for nearly 50 years, and the purpose of this article is to look at some of the stages that such lenses have gone through in reaching their present state of development. One of the most useful sources of information on such lenses is the patent literature, which can be accessed freely via the internet. US patents are particularly easy to search (www.uspto.com),  while the European patent office has an extremely comprehensive site for worldwide patents (www.espacenet.com).

Up to the beginning of April 2006, the US Patent Office had issued 263 patents over the years specifically relating to progressive power spectacle lenses, with other applications pending.

 Historically, the patent of Aves in 1907 for a progressive power lens, and a manufacturing method, is the first that can be identified for a practical lens. One of the features of this lens was that the power did not stabilise for distance or near vision, and also there was no obvious method in the manufacturing process for incorporating a cylindrical prescription. The progression was achieved by using two conic aspheric surfaces.

Despite many other attempts over the subsequent years to produce a  practical lens, it was the French company Société  Industrielle et Commerciale des Ouvriers Lunetiers (now Essilor) which developed the first commercially successful progressive lens, the original Varilux.
This was designed by Bernard Maitenaz, and showed the now common features of general purpose lenses:

1) Large stable distance vision area in the top part of the lens
2) A small area of stable near vision power in the lower part of the lens
3) A progressive power corridor smoothly joining 1 and 2, flanked by areas of unwanted surface astigmatism
4) Complete 'invisibility' in that there are no visible margins to the power zones.

Requirement 4 can be met by the blended bifocal (Figure 1), which had some popularity in the US, however these lenses have no clear progression from distance to near power.

When first introduced in 1959, Varilux was only available in one material, white ophthalmic crown glass, and in a limited range of prescriptions. It was not until  1966 that a CR39 version of Varilux became available.The next manufacturers to start promoting lenses in the UK were BBGR (with the 'Zoom' lens) and American Optical with 'Ultravue'. There was also a certain amount of 'badge engineering' where some lenses appeared under different names from different suppliers.

Arguably, the most significant design step was the introduction of Varilux 2. This lens could be considered the first of the modern designs, as it used a progressive front surface with aspheric horizontal sections. This lens, unlike predecessors, allowed the peripheral aberrations to extend into the periphery of the distance area. The fact that the surface power change was now allowed to spread over a larger area than before meant that the surface astigmatism was less than hitherto. In addition, the start of the progression was less abrupt, which meant that there was a little more tolerance on the vertical fitting position. This gave rise to the terms 'hard' and 'soft' lenses, these relating to how quickly the progression developed from the distance prescription, with Varilux 2 being the first of the so-called 'soft' designs. It is perhaps worth mentioning that modern designs would all be considered of the 'soft' variety and are more of a compromise.

Varilux 2 also used horizontal aspheric sections to try to control the distortion caused by the increased magnification in the progression zone of the lens. In a bifocal, this increased magnification gives rise to lateral image jump, but in a progressive addition lens you get skew distortion. By using varying aspheric curves on the front progressive surface, the Varilux 2 artificially increased the peripheral magnification in the lower power distance curve, and progressively reduced the peripheral magnification in the increased power progressive zone, to try and balance out these effects.

One feature of very early lenses was a single design was produced for both eyes which was then rotated to give a near inset. In 1970, Société des Lunetiers patented the concept of having separate right and left lenses, with the near zone being inset relative to a horizontal reference. Binocular vision was also of importance, and Zeiss paid particular attention to this in the design of its Gradal HS lens.

Surface astigmatism had seemed to be the bugbear of progressive lens design, but the fashion for large eyesize spectacle frames in the 1980s gave lens designers the opportunity to try longer progressions. The advantage of a longer progression is that the rate of change of power can be spread over a larger area in the periphery of the lens, and in simplistic terms, surface astigmatism is related to the rate of change of power.

Perhaps the most extreme example of this was the American Optical 'Omni' lens, where the low level of aberrational astigmatism is gained at the expense of a narrowing of the stable distance power zone. Unfortunately, frame fashions again returned to smaller lens sizes, so that long progressions could not so readily be used.

How we assess lenses

One of the major difficulties that always faces an optometrist or dispensing optician is that of assessing new lenses. Apparatus for making power maps or other objective assessment is not widely available, and then there is the problem of interpretation of the data. At the end of the day, a practitioner is always going to ask, 'Can my patients wear this lens?' Inevitably there is a tendency for practitioners to rely on lenses with which they are familiar. Ideally, we would like to do wearer trials on all new lenses, but these are expensive and time consuming to carry out.

However, the continuous feedback from wearers is of value to manufacturers in refining progressive lens designs, and is a valuable and integral part of the development process. Thus, a wearer trial may show up a particular problem with a certain addition or specific part of the power range. Inevitably, we have to fall back on objective measurement of the power distribution across the lenses, usually displayed as contour plots.

Calculation methods

Early lenses would have been designed essentially by hand, using slide rules and desk top calculators. But modern personal computers can now do the calculation work in a fraction of the time. This has led to more accurate simulations being possible, and design work looking beyond the traditional aberrations. Thus, the recently announced Essilor Varilux 'Physio' lens claims to control the aberration coma in the distance zone.

Improved manufacture

Running in parallel with improvements in lens design has been a similar rapid development in manufacturing techniques. This has got to the point where it is economic (albeit for a premium product) to manufacture a progressive surface individually to prescription. This means that the surface also incorporates the prescription cylinder. This is a considerable departure from the traditional method of manufacturing semi-finished progressive lenses, with the prescription being incorporated into a spherical or toroidal second surface. This has all been made possible by the use of numerically controlled machine tools (CNC machines) which can cut any smooth surface that can be defined mathematically. In addition, developments in the polishing of materials have ensured that subtleties in design are not removed in the polishing process.

Wider range of materials

When the original Varilux was introduced, it was only in one material. Nowadays, it is expected that a wide range of refractive indices is available, and also photochromic materials. This can cause problems for the manufacturer, as it can be difficult to exactly reproduce the same design by sometimes very different manufacturing methods. Figure 2 illustrates power measurements taken on a line across three lenses of the same type and prescription, but in different materials.

It is also a feature of modern lenses that they are flatter in form. A glass Varilux 2 with a plano/+2.00 add prescription had a front surface curve of +6.75D across the reference circles, whereas a modern Varilux Panamic has a value of +4.50D. This, of course, improves the cosmetic appearance as the lens is less bulbous, but has come about because the use of improved designs of aspheric curves have enabled flatter forms to be produced with acceptable optics.

One area where lenses have not perhaps developed as much as some might have hoped is in the prescription range. This is not a problem in minus distance prescriptions, but in the plus, most designs are limited to +6.00D or thereabouts. Only one design (Varilux Omega) has been made available for high plus prescriptions.

Most lenses are not available in additions over +3.50D due to the inevitable compromises that have to be made in terms of optical quality for such high powers. Norville now supplies a lens with a +4.00 addition, the NSP4. Designing lenses with such a high addition is a challenge, and to give an indication of this, one (obsolete) design of lens with a +3.50 addition was measured and showed a maximum value of aberrational astigmatism (plano distance) of 7.2 dioptres at 15mm lateral to the progression!

In the 1990s, the fashion for smaller frames led to a demand for shorter progression lengths, with lenses such as the American Optical 'Compact' being produced. The variation in mean power down the centre of such a lens is illustrated in Figure 3.

Special purpose lenses

A wide variety of special purpose designs have been developed over the years for specific purposes. Lenses such as the Varilux Pilot with a bifocal segment in the upper part of the lens, American Optical Technica for intermediate use, and Rodenstock also developed a lens specifically for half frame spectacles.

Perhaps the most successful types have been those designs with an enhanced intermediate zone, designed specifically for computer users. Figure 4 shows a modern example of such a lens, the Varilux Computer 3V. Note the wide corridor, achieved by using a long progression. Figure 3 also illustrates the small area of stable distance power that is typical of such lens types, which are designed specifically for intermediate and near use.

'Individual' designs, Rear surface designs

These recent developments have been made possible by the developments mentioned earlier of machinery that can manufacture an individual lens to prescription. Seiko-Epson introduced a lens where the front surface was rotationally symmetrical spherical surface, with the rear surface incorporating a complex combined cylinder and progression. The claimed benefits of this are smaller differences in magnification when changing from distance to near vision, as well as a wider near field of clear vision. Rodenstock introduced a lens where individual patent data could be incorporated into a design, and other manufacturers also are now offering lenses produced to an individual specification.
In some respects, the design of general purpose progressives has come full circle from Aves, in that there are now lenses where the addition is produced by the sum of both surfaces. For example, lenses from Johnson & Johnson and Hoya.

What will happen next?

In view of the limited number of parameters available to the designer of a progressive power spectacle lens, it is perhaps surprising that new lenses are still being developed. Designers are continuously searching for a new set of compromises in lens specification which will give a competitive edge, and of equal importance, are features that can be marketed to the ophthalmic professions and the end users. Having measured the wavefront aberrations of a number of designs, Villegas and Artal concluded that 'the tested progressive power lenses perform like a waterbed, where the astigmatism is the water that can be moved, but not eliminated'.15 So new lenses will continue to be produced in an attempt to find designs that are acceptable to as wide a range of wearers as possible.

References
1 Aves O. 'Improvements in and relating to multifocal lenses and the like.' UK Patent 15,735 (1907).
2 Société Industrielle et Commerciale des Ouvriers Lunetiers. 'Improvement in or relating to optical systems having a locally variable power.' UK Patent 790,310 (1958).
3 Maitenaz B. 'Variplas, the plastics version of Varilux'. The Ophthalmic Optician,1966; 909-927.
4 Société des Lunetiers. 'Improvements in or relating to ophthalmic lenses with progressively varying focal length'. UK Patent 1,212,543 (1970).
5 Fu‘ter G and Lahres H. 'Multifocal spectacle lens with a dioptric power varying progressively between different zones of vision' US Patent 4,606,622 (1986).
6 Winthrop JT. 'Progressive addition spectacle lens' US Patent 4,861,153 (1989)
7 Fowler CW, Thompson A and Bartlam E. 'A wearer trial of two rear surface progressive addition spectacle lenses'. Optometry in Practice, 2002; 3, 51-53
8 Fowler, CW 'Technical note: apparatus for comparison of progressive addition spectacle lenses'. Ophthalmic Physiol Opt, 2006; In press.
9 Winthrop JT. 'Short-corridor progressive lens'. US Patent 6,142,627 (2000).
10 Barth R. 'Spectacle lens for half-eye spectacles'. US Patent 4,906,090 (1990).
11 Mukaiyama H and Kato K. 'Progressive multifocal lens and manufacturing method of eyeglass lens and progressive multifocal lens'. US Patent 6,019,470 (2000).
12 Baumbach P, Esser G, Mueller W, Brosig J, Haimerl W, Altheimer H, Pfeiffer H. 'Method of manufacturing progressive ophthalmic lens'. US Patent 6,685,316 (2004).
13 Menezes EV 'Progressive addition lens having regressive surfaces'. US Patent 6,139,148 (2000)
14 Kitani A and Kikuchi Y. 'Bi-aspherical type progressive power lens'. US Patent 6,935,744 (2005).
15 Villegas EA and Artal P. 'Comparison of aberrations in different types of progressive power lenses'. Ophthalmic Physiol Opt, 2004; 24, 419-426.

Colin Fowler is a senior lecturer at Aston University

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